Field of the Invention
The present invention relates to a floor panel which is used in a free access floor.
Description of the Conventional Art
The floor panel according to the present invention is used in the free access floor. The floor panel of the free access floor is provided above a foundation floor surface which is formed by a concrete material and constructs a part of a building construction, so as to be spaced (refer, for example, to patent document 1).
FIGS. 8 to 11 are views which are referred for describing a conventional floor panel 2. The conventional floor panel 2 constructs a free access floor 1 together with a support leg 4 which is provided in a rising manner on a foundation floor surface 3, as shown in FIG. 8.
The floor panel 2 is arranged on a horizontal surface having a height spaced above from the foundation floor surface 3 so as to be adjacent to each other in both vertical and horizontal axial directions as seen from the above, and four corner portions of the floor panel 2 are detachably supported to the support legs 4 which are provided in a rising manner on the foundation floor surface 3.
The floor panel 2 is constructed as shown by FIGS. 9 and 11, by mainly combining a top plate 6 formed approximately as a tabular shape so that an upper surface 6c is formed approximately as a square shape, and a bottom plate 8 formed as a box shape having a bottom surface 8c which is approximately formed as a square shape in the same manner as the upper surface 6c and having a depth, vertically so that a top plate portion 6a and a bottom plate portion 8a are opposed to each other, as shown in FIG. 10. The top plate 6 and the bottom plate 8 are both constructed by using a steel plate member.
In the bottom plate 8 of the floor panel 2, as shown in FIG. 11, there is formed a side plate portion 8b obtained by folding the bottom plate portion 8a approximately vertically from a peripheral edge portion toward an upper side in the drawing, and a peripheral edge portion 8f folded approximately vertically toward a right side in the drawing is formed in an upper end portion of the side plate portion 8b. As a result, the bottom plate 8 is formed as a box shape having a concave space 8d which is formed in an inner side in a horizontal direction of the side plate portion 8b. 
Further, since a U-shaped portion 6b folded like a U-shaped form so as to enwrap the peripheral edge portion 8f of the bottom plate 8 is formed in the peripheral edge portion 6f of the top plate 6, the peripheral edge portion 6f of the top plate 6 is connected to the peripheral edge portion 8f of the bottom plate 8.
As mentioned above, the floor panel 2 is formed as a hollow shape having an internal space 10 by integrally connecting the top plate 6 and the bottom plate 8. Further, the internal space 10 is provided with a reinforcing member 12 having a thickness.
However, since the bottom plate 8 is obtained by a deep drawing process of a plate-like member while using a pressing machine, such a tension force that the bottom plate 8 is extended in a depth direction (a vertical direction in FIG. 11) is applied. Therefore, there is a problem that a crack or a breaking may be easily generated in the side plate portion 8b of the bottom plate 8 in the case that the thickness becomes smaller.
FIGS. 9 and 12 to 14 are views which are referred for describing a conventional floor panel 20. The conventional floor panel 20 is constructed, as shown in FIG. 9, by mainly combining a top plate 26 in which an upper surface 26c is formed approximately as a square box shape, and a bottom plate 28 in which a bottom surface 28c corresponding to the upper surface 26c is formed approximately as a square box shape, vertically as shown in FIG. 12.
A side plate portion 26b is formed in the top plate 26 of the floor panel 20, the side plate portion 26b being formed by folding the top plate portion 26a approximately vertically from a peripheral edge portion thereof to a lower side in the drawing, and a side plate portion 28b is formed in the bottom plate 28 of the floor panel 20, the side plate portion 28b being formed by folding the bottom plate portion 28a approximately vertically from a peripheral edge portion thereof toward an upper side in the drawing, as shown in FIG. 13.
As a result, the top plate 26 is formed as a box shape having a concave space 26d which is formed in an inner side in a horizontal direction of the side plate portion 26b. Further, the bottom plate 28 is also formed as a box shape having a concave space 28d which is formed in an inner side in a horizontal direction of the side plate portion 28b. 
Further, in the floor panel 20, concave spaces 26d and 28d of the top plate 26 and the bottom plate 28 are arranged so as to be communicated with each other, and a peripheral edge portion 26j and a peripheral edge portion 28j are folded as a U-shaped form so as to enwrap respective leading end portions, thereby being connected in a joint portion 25 approximately in the center portion in a vertical direction in FIG. 13 of the floor panel 20, the peripheral edge portion 26j being formed in a lower end portion side of the side plate portion 26b of the top plate 26, and the peripheral edge portion 28j being formed in an upper end portion side of the side plate portion 28b of the bottom plate 28.
As mentioned above, since the top plate 26 and the bottom plate 28 are integrally connected in the joint portion 25, the floor panel 20 is formed as a hollow shape having an internal space 30. Further, a reinforcing member 32 having a thickness is provided in the internal space 30.
As mentioned above, since it is possible to make depths of the concave space 26d of the top plate 26 and the concave space 28d of the bottom plate 28 approximately half of a depth of the concave space 8d of the bottom plate in the floor panel 2, it is possible to make a height at which the side plate portions 26b and 28b are extended by a deep drawing process smaller than a height at which the side plate portion 8b of the bottom plate 8 is extended by a deep drawing process, at that degree.
As a result, since it is possible to reduce a tension force which is applied at the time of the deep drawing process of the top plate 26 and the bottom plate 28, it is possible to prevent a crack and a breaking from being easily generated at the time of the forming process even in the case that the thicknesses of the top plate 26 and the bottom plate 28 are reduced.